The present invention relates to a noncontact tonometer by which intraocular pressure of a patient""s eye is measured, without contacting the eye, by directing a compressed air pulse into a cornea of the patient""s eye and deforming the cornea in a predetermined manner. More particularly, the present invention relates to a noncontact type tonometer in which intraocular pressure of the patient""s eye can be precisely measured without being affected by atmospheric changes or giving discomfort to the patient.
A noncontact type tonometer, in which intraocular pressure of the patient""s eye is measured by directing compressed air to the cornea of the patient""s eye and applanating thereof, is well-known. U.S. Pat. No. 3,585,849, for example, discloses a noncontact tonometer in which the intraocular pressure of the patient""s eye is measured based on an elapsed time interval during which a surface of the cornea deforms into a flat state from a convex state while being deformed by a compressed air pulse directed thereto. In the noncontact type tonometer mentioned above, the pressure of the air pulse necessary to deform the cornea to the flat state is indirectly obtained according to the time interval elapsed during which the surface of the cornea deforms into the flat state from the convex state, assuming that the pressure of the air pulse is controlled correctly based on the optimum control condition.
Additionally, in Japanese Patent Application after substantive examination, Laid-open No. SHO 63-58577, a noncontact type tonometer in which, the pressure of the air pulse corresponding to the flat state of the cornea is directly obtained from a pressure sensor arranged in a device for producing the air pulse.
However, in the former tonometer, measuring accuracy thereof depends on whether or not the air pulse producing device is controlled under the predetermined optimum control condition. Therefore, the measuring accuracy of the air pulse pressure in the tonometer will not be reliable unless the air pulse producing device is controlled under the predetermined optimum control condition.
The determining method of the air pulse pressure according to the above tonometer will be described based on FIG. 5. FIG. 5 is a graph to explain the determining method of the air pulse pressure in the conventional tonometer, in which the ordinate shows the air pulse pressure, the abscissa shows time, f is a standard curvature which shows change of the air pulse pressure versus time (abbreviated xe2x80x9cstandard p-t curvaturexe2x80x9d hereinafter), g is an actually measured curvature with various error which shows change of the air pulse pressure versus time (abbreviated xe2x80x9cactual p-t curvaturexe2x80x9d hereinafter), h is a curvature which shows change of light quantity reflected from the cornea and maximum light quantity is obtained at a time t because the cornea deforms to the flat state at the time t, p(t) is the air pulse pressure obtained from the standard p-t curvature f at the time t, txe2x80x2 is a time at which the maximum light quantity will be obtained if the air pulse pressure is measured without any error and pxe2x80x2(t) is the air pulse pressure obtained from the standard p-t curvature f at the time txe2x80x2.
According to FIG. 5, the air pulse pressure is measured based on a condition (such condition is deviated from the predetermined optimum condition) including various errors such as atmospheric change surrounding the tonometer, mechanical error produced in a piston mechanisms of the air pulse producing device, the air pulse pressure is determined as p(t) at the time t according to the standard p-t curvature f in FIG. 5, since the air pulse pressure is obtained based on the time t at which the maximum light quantity reflected from the cornea is detected because of the flat state thereof. Thereafter, the intraocular pressure of the patient""s eye is calculated from the air pulse pressure p(t). Such obtained air pulse pressure p(t) deviates from the air pulse pressure pxe2x80x2(t) to be obtained without any error at the time txe2x80x2. As mentioned above, a defect in the measuring error of the air pulse pressure will be caused by the above various errors which exist in the conventional tonometer.
In the latter tonometer, the measuring error of the air pulse pressure is not caused by the atmospheric change or the mechanical error mentioned above, and reproducibility in measuring of the air pulse pressure is good because the intraocular pressure of the patient""s eye is calculated based on the air pulse pressure directly detected by the pressure sensor at the time when the maximum light quantity reflected from the cornea is obtained. However, in the latter tonometer, after the air pulse pressure is produced, the change of the air pulse pressure versus time is not monitored, therefore, early detection of the abnormal state in the air pulse pressure caused by a malfunction of the piston in the air pulse pressure producing device or binding in a nozzle part formed in the air pulse pressure producing device to direct the compressed air pulse to the cornea of the patient""s eye cannot be conducted. Further, excessive air pulse pressure may be directed to the cornea of the patient""s eye, giving discomfort to the patient, since the change in the air pulse pressure versus time is not monitored in the latter tonometer.
Accordingly, it is an object of the present invention to overcome the above-mentioned problems and to provide a noncontact type tonometer in which the air pulse pressure can be correctly measured without being affected by various errors such as the atmospheric change and the mechanical error caused in the piston mechanism in the air pulse pressure producing device and the application of excessive air pulse pressure to the cornea of the patient""s eye can be avoided.
To accomplish the above object, the present invention comprises a noncontact type tonometer having a compressing means for forming compressed air, a direction means for directing the compressed air to a cornea of a patient""s eye and a deformation detection means for detecting deformation state of the cornea to which the compressed air is directed by the direction means, the noncontact type tonometer further comprising: a memory means for storing a standard pressure characteristic of the compressed air which changes according to passage of time, a pressure detection means for detecting pressure change of the compressed air directed by the direction means, a time detection means for detecting the time elapsed until the deformation detection means detects a flat state of the cornea, a comparison means for comparing the pressure change of the compressed air detected by the pressure detection means with the standard pressure characteristic stored in the memory means, an amendment means for amending the time detected by the time detection means according to the standard pressure characteristic based on result compared by the comparison means, and a first calculation means for calculating an intraocular pressure of the patient""s eye based on the amended time by the amendment means.
In the present invention, the compressed air is directed to the cornea of the patient""s eye by the direction means. During direction of the compressed air, the shape of the cornea is deformed from the convex state to the flat state, and while the cornea is deforming, the deformation state of the cornea is detected by the deformation detection means and the pressure change of the compressed air is detected by the pressure detection means, and further, the time elapsed until the deformation detection means detects the flat state of the cornea is detected by the time detection means.
Next, the comparison means compares the pressure change detected by the pressure detection means with the standard pressure characteristic stored in the memory means. Following this comparison by the comparison means, the time detected by the time detection means is amended according to the standard pressure characteristic based on the compared result. Thereafter, the intraocular pressure of the patient""s eye is calculated based on the amended time by the first calculation means.
Therefore, according to the present invention, fluctuation of the intraocular pressure can be prevented, thus, the precise intraocular pressure can be stably obtained, because delicate change in the measured air pressure caused due to the atmospheric change or the mechanical error can be eliminated.
Further, abnormal rises in the air pressure can be prevented, thus, discomfort will not be given to the patient, because the air pressure changing state is continuously monitored.